The present invention relates to a flight control system for a hybrid aircraft, and more particularly, to a flight control system for a hybrid unmanned aerial vehicle (UAV) which blends command signals to a multiple of vehicle control surfaces during transition between rotor borne and wing borne flight.
There is an increased emphasis on the use of UAVs for performing various activities in both civilian and military situations where the use of manned flight vehicles may not be appropriate. Such missions include surveillance, reconnaissance, target acquisition, target designation, data acquisition, communications relay, decoy, jamming, harassment, ordinance delivery, or supply.
A hybrid aircraft provides the hover and low-speed maneuverability of a helicopter with the high-speed forward flight and duration capabilities of a winged aircraft. Typically, hybrid aircraft include a helicopter control surface system which provides cyclic pitch, collective pitch and differential rotation to generate lift, pitch, roll, and yaw control when operating in a hover/low-speed environment. Additionally, the hybrid aircraft includes a conventional fixed wing aircraft control surface system such as aileron, elevator, rudder and flaps to provide control when operating in a high-speed environment. Hybrid aircraft also typically include a separate translational propulsive system.
When the hybrid aircraft is operating in a hover/low-speed environment, maneuverability is achieved by controlling the helicopter control system. When the hybrid aircraft is operating in a high-speed environment, the hybrid aircraft operates as a fixed wing aircraft and maneuverability is achieved by controlling the aircraft flight control surfaces. As the hybrid aircraft transitions between helicopter and aircraft control surface systems, however, neither the helicopter nor the aircraft control systems are completely effective. Moreover, the relationship between control displacement and control moment is nonlinear and the aerodynamic forces on the aircraft change most dramatically. Flight control within this region is therefore rather complex.
Accordingly, it is desirable to provide a hybrid aircraft flight control systems which automatically blends command signals to a multiple of vehicle flight control surfaces during transition between rotor borne and wing borne flight. It is further desirable to provide the same amount of vehicle control regardless of the vehicle""s flight regime.
Hybrid aircraft include a flight control system according to the present invention. A hybrid aircraft can hover like a helicopter using a rotor system or fly like a fixed wing aircraft using conventional fixed wing controls such that it is operable in four flight regimes:
1. Hoverxe2x80x94Defined as low speed operation. The rotor generates control and lift.
2. Forward Flightxe2x80x94Lift is generated by the wings and all control is through the fixed wing surfaces (elevator, ailerons, rudder)
3. Transition Upxe2x80x94This mode guides operation of a multiple of control surfaces when flying from Hover to Forward Flight.
4. Transition Downxe2x80x94This mode guides operation of a multiple of control surfaces when flying from Forward Flight to Hover.
The flight control system according to the present invention includes a blending algorithm which evaluates the current flight regime and determines the effectiveness of the various flight control surfaces. In the yaw axis, gain schedules for both differential collective (rotor) and rudder control are used as a quantitative measure of control effectiveness. Based on the respective gain schedules, the blending algorithm determines how much of an input command is sent to each control surface. The result is that for a given command, the same amount of yaw moment will be generated regardless of flight regime. This simplifies the underlying flight control laws since the commands it generates are correct regardless of flight regime.
The present invention therefore provides a hybrid aircraft flight control system which automatically blends yaw command signals between differential collective and rudder flight control surfaces during transition between rotor borne and wing borne flight.